1. Field of the Invention
The invention relates to a fuel cell system, and in particular, relates to art for controlling an amount of fuel off-gas discharged from a fuel cell system to outside.
2. Description of the Related Art
A fuel cell has a structure in which an anode and a cathode are arranged with an electrolyte membrane therebetween. An electrochemical reaction occurs when fuel gas including hydrogen contacts the anode, and oxidation gas including oxygen from air or the like contacts the cathode, thus generating an electromotive force. For a fuel cell system that obtains electric power using such a fuel cell, eliminating excessive consumption of hydrogen fuel is important from the standpoint of energy conservation. To this end, a fuel cell system such as that disclosed in, for example, Japanese Patent Application Publication No. JP-A-2002-289237, circulates off-gas of fuel gas used by the fuel cell through the fuel cell again in order to effectively use hydrogen remaining in the fuel off-gas.
Moreover, fuel gas supplied to the anode and oxidation gas flowing to the cathode are separated by the electrolyte membrane. A portion of the fuel gas passes through the electrolyte membrane to the cathode side; conversely, a portion of the oxidation gas passes through the electrolyte membrane to the anode side. The oxidation gas emerging on the anode side includes fuel off-gas and is circulated through the fuel cell. However, the main component of the oxidation gas is inert nitrogen, which builds up in the fuel gas circulation system, thus gradually increasing the concentration of nitrogen in the fuel gas supplied to the fuel cell. This increase in the nitrogen concentration of fuel gas also suggests a decrease in the hydrogen concentration, which may lower the power generation performance of the fuel cell.
With regard to this point, art disclosed in Japanese Patent Application Publication No. JP-A-2002-289237 opens a valve communicating with a fuel off-gas passage for a predetermined period, when the concentration of impurities in the circulating fuel gas reaches an intolerable level. Thus, nitrogen accumulated in the circulation system is discharged from the circulation system together with fuel off-gas, allowing the restoration of the hydrogen concentration in the fuel gas supplied to the fuel cell.
However, the open time of the valve is fixed in the above art, and therefore, a sufficient amount of nitrogen may not be discharged depending on the state of accumulated nitrogen in the circulation system. Thus, the hydrogen concentration may not adequately recover. Furthermore, hydrogen is also discharged from the circulation system together with nitrogen in the discharge of fuel off-gas. By leaving the valve open for a fixed period, as in the above art, more hydrogen than necessary may be discharged while the hydrogen concentration is restored, which may also lower fuel economy. Accordingly, when fuel off-gas is discharged outside the circulation system to restore the hydrogen concentration, the amount of discharged fuel off-gas must be accurately controlled so that hydrogen is not unnecessarily discharged.
In addition, the decrease in the fuel gas hydrogen concentration caused by nitrogen passing through the electrolyte membrane is not limited to only the type of fuel cell system that operates while circulating fuel gas as described above. For example, as disclosed in Japanese Patent Application Publication No. JP-A-2002-8691, this problem is also shared with anode dead end-type fuel cell systems that operate with the outlet of the fuel gas passage to the anode closed. In an anode dead end-type fuel cell system, fuel gas is supplied in accordance with the consumption of hydrogen on the anode side. However, since nitrogen passes through the electrolyte membrane and accumulates in the fuel gas passage to the anode, the hydrogen concentration in the anode gradually decreases. As a result, even in such an anode dead end-type fuel cell system, nitrogen accumulated in the fuel gas passage to the anode is discharged together with fuel off-gas outside the system, and an art for accurately controlling the amount of discharge at that time is required.